In the manufacture of aluminum foils or sheets, conventional twin roll continuous sheet casting machines comprise a pair of parallel, water-cooled, counter-rotating rolls. After being in use for a given period, the surface of the roll must be reground because of heat cracks resulting from thermal fatigue and out-of-roundness due to galling between caster core and shell. Accordingly, the shells surrounding the cores must be removed periodically, and the cores repaired and reground before rebuilding of the roll assembly.
The major cause of damage to caster roll assemblies is galling between the core and the shell, which occurs when the shell slips relative to the core under load. Cold welding of the core and shell during relative motion causes metal to be torn from the core and displaced in the interface between the two, resulting in distortion of the rolls and roll gap, creating bad shape in the continuously cast sheet.
Using such conventional roll assemblies, it is possible to cast from about 10 to 12 millions pounds of aluminum sheet before cutting of the shells and repairing and regrinding the cores. However, after regrinding two or three times, the hardened layer on the core surface is lost and the cores are destroyed by deformation. To overcome this problem, it became general practice to coat cores with a stainless steel overlay. This modification extended the service life between regrinding by about 50% and avoided destruction of cores by weld rebuilding to original dimensions and regrinding. However, the stainless steel weld overlay is still subject to cold welding and galling between the shell and the core.
Another approach to extend the service life of the core is to coat the inner surface of the shell with hard chromium, as proposed in U.S. Pat. No. 5,265,332, which is hereby incorporated by reference. Again, however, usual problems with the stainless steel overlay are present.
There is therefore a great need to develop a core having an extended service life between shell removal. Preferably, the core would be prepared by simple techniques at a reasonable cost.